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Bridge Scour and Stream Instability Countermeasures: Experience, Selection, and Design Guidance-Third Edition
Design Guideline 10 Gabion Mattresses
Gabion mattresses are containers constructed of wire mesh and filled with rocks. The length of a gabion mattress is greater than the width, and the width is greater than the thickness. Diaphragms are inserted widthwise into the mattress to create compartments (Figure 10.1). Wire is typically galvanized or coated with polyvinyl chloride to resist corrosion, and either welded or twisted into a lattice. Stones used to fill the containers can be either angular rock or rounded cobbles; however, angular rock is preferred due to the higher degree of natural interlocking of the stone fill. During installation, individual mattresses are connected together by lacing wire or other connectors to form a continuous structure. Figure 10.2 shows the installation of a gabion mattress system.
The wire mesh allows the gabions to deform and adapt to changes in the subgrade while maintaining stability. Additionally, when compared to riprap, less excavation of the bed is required and smaller, more economical stone can be used. The obvious benefit of gabion mattresses is that the size of the individual stones used to fill the mattress can be smaller than stone that would individually be too small to withstand the hydraulic forces of a stream (Freeman and Fischenich 2000). This design guideline considers two applications of gabion mattresses: Application 1 - bank revetment and bed armor; and Application 2 - pier scour protection.
There is limited field experience with the use of gabion mattresses systems as a scour countermeasure for bridge piers alone. More frequently, these systems have been used for structures such as dams or dikes, or for channel slope stabilization. The guidance for pier scour applications provided in this document has been developed primarily from Federal Highway Administration (FHWA) Hydraulic Engineering Circular No. 23 (HEC-23) (Lagasse et al. 2001b), NCHRP Project 24-07(1) (Parker et al. 1998), and NCHRP Project 24-07(2) (Lagasse et al. 2007). Durability of the wire mesh under long term exposure to the flow conditions at bridge piers has not been demonstrated; therefore, the use of gabion mattresses as a bridge pier scour countermeasure has an element of uncertainty (Parker et al. 1998).
Successful long-term performance of gabion mattresses depends largely on the integrity of the wire. Due to the potential for abrasion by coarse bed load, gabion mattresses are not appropriate for gravel bed streams and should only be considered for use in sand- or fine-bed streams. Additionally, water quality of the stream must be noncorrosive (i.e., nonsaline and nonacidic). A polyvinyl chloride (PVC) coating should be used for applications where the potential for corrosion exists.
Standard test methods relating to material type, characteristics, and testing of rock and aggregates typically associated with riprap installations (e.g., filter stone and bedding layers) are provided in this section and are recommended for specifying the rock fill used in gabion mattresses. In general, the test methods recommended in this section are intended to ensure that the stone is dense and durable, and will not degrade significantly over time.
Rocks used for gabion mattresses should only break with difficulty, have no earthy odor, no closely spaced discontinuities (joints or bedding planes), and should not absorb water easily. Rocks comprised of appreciable amounts of clay, such as shales, mudstones, and claystones, are never acceptable for use as fill for gabion mattresses. Table 10.1 summarizes the recommended tests and allowable values for rock and aggregate.
(1) Testing frequency for acceptance of riprap from certified quarries, unless otherwise noted. Project-specific tests exceeding quarry certification requirements, either in performance value or frequency of testing, must be specified by the Engineer.
Successful gabion performance depends not only on properly sizing and filling the baskets, but also on the quality and integrity of the wire comprising the basket compartments, diaphragms, lids, and lacing wire. Investigations conducted under NCHRP Project 24-07(1) (Parker et al. 1998) concluded that the lacing wire in particular proved to be the weakest link of gabion mattress systems. Wire should be single strand galvanized steel; a PVC coating may be added to protect against corrosion where required.
The wire mesh may be formed with a double twist hexagonal pattern or can be made of welded wire fabric. Fasteners, such as ring binders or spiral binders, must be of the same quality and strength as that specified for the gabion mattresses. The following recommendations are provided for twisted-wire and welded-wire gabions, respectively:
Twisted-Wire Gabion Mattresses: A Producer's or Supplier's certification shall be furnished to the Purchaser that the material comprising the gabion mattress components and lacing wire was manufactured, sampled, tested, and inspected in accordance with the specifications of: ASTM A 975, "Standard Specification for Double-Twisted Hexagonal Mesh Gabions and Revet Mattresses (Metallic-Coated Steel Wire or Metallic-Coated Steel Wire with Poly Vinyl Chloride (PVC) Coating)."The certification must indicate that the minimum requirements of this standard have been met.
Welded-Wire Gabion Mattresses: A Producer's or Supplier's certification shall be furnished to the Purchaser that the material comprising the gabion mattress components and lacing wire was manufactured, sampled, tested, and inspected in accordance with the specifications of: ASTM A 974, "Standard Specification for Welded Wire Fabric Gabions and Gabion Mattresses (Metallic-Coated or Poly Vinyl Chloride (PVC) Coated)." The certification must indicate that the minimum requirements of this standard have been met.
Flexibility of the gabion mattress units is a major factor in the successful performance of these systems. The ability to adjust to differential settlement, frost heave, or other changes in the subgrade is desirable. For example, settlement around the perimeter of a gabion mattress installation at a bridge pier is beneficial if scour at the edges of the mattresses occurs. Rigid systems are more prone to undermining and subsequent damage to the mesh, and are therefore less suitable for use at bridge piers. Designers are encouraged to further familiarize themselves with the flexibility and performance of various gabion mattress materials and proprietary products for use in riverine environments.
10.3 APPLICATION 1: HYDRAULIC DESIGN PROCEDURE FOR GABION MATTRESSES FOR BANK REVETMENT OR BED ARMOR
Gabion mattress design methods typically yield a required d50 stone size that will result in stable performance under the design hydraulic loading. Because stone is produced and delivered in a range of sizes and shapes, the required size of stone is often stated in terms of a minimum and maximum allowable size. For example, ASTM standard D 6711, "Standard Practice for Specifying Rock to Fill Gabions, Revet Mattresses, and Gabion Mattresses," recommends the following:
ASTM standard D 6711 also indicates that the fill should be well graded with a full range of sizes between the upper and lower limits. The rocks used to fill gabion mattresses should be hard, dense, and durable. In general, rocks used for filling gabion mattresses should be of the same material quality as would be used for riprap, as described in Design Guide 4 of this document.
The designer must determine what factor of safety should be used for a particular application. Typically, a minimum allowable factor of safety of 1.2 is used for revetment (bank protection) when the project hydraulic conditions are well known and the installation can be conducted under well-controlled conditions. Higher factors of safety are typically used for protection at bridge piers, abutments, and at channel bends due to the complexity in computing hydraulic conditions at these locations.
The Harris County Flood Control District, Texas (HCFCD 2001) has developed a simple flow chart approach that considers the type of application, uncertainty in the hydraulic and hydrologic models used to calculate design conditions, and consequences of failure to select an appropriate target factor of safety to use when designing various types of Articulating Concrete Block (ACB) installations. In this approach, the minimum allowable factor of safety for ACBs at bridge piers, for example, is 1.5. This base value is then multiplied by two factors, each equal to or greater than 1.0, to account for risk and uncertainty. Figure 10.3 shows the HCFCD flow chart method. The method is also considered appropriate for gabion mattresses, since the design method results in a calculated safety factor.
For gabion mattresses placed on channel beds or banks, the shear stress on the mattress is calculated as follows:
The bend coefficient Kb is used to calculate the increased shear stress on the outside of a bend. This coefficient ranges from 1.05 to 2.0, depending on the severity of the bend. The bend coefficient is a function of the radius of curvature Rc divided by the top width of the channel T, as follows:
The recommended procedure for determining the permissible shear stress for a gabion mattress is determined using the relationship provided in Hydraulic Engineering Circular No. 15 (HEC-15) third edition (Kilgore and Cotton 2005):
The coefficient Cs is an empirical coefficient developed by Maynord (1995) from test data presented in Simons et al. (1984). Use of Cs = 0.10 is limited to the conditions of the testing program, which used angular rock and a ratio of maximum to minimum stone size from 1.5 to 2.0.
The Factor of Safety can be calculated as the ratio of the permissible shear stress divided by the applied shear stress:
Minimum rock size should be at least 1.25 times larger than the aperture size of the wire mesh that comprises the mattress (Parker et al. 1998). Rock should be well graded between the minimum and maximum sizes to minimize the size of the voids in the matrix. If design criteria and economic criteria permit, standard gradations may be selected.
The thickness of the gabion mattress should be at least twice the average diameter of the rock fill, T ≥ 2 d50. If the computed thickness does not match that of a standard gabion thickness, the next larger thickness of mattress should be used (Maynord 1995). At a minimum, the thickness should be 6 in. (Parker et al. 1998).
Longitudinal Extent: The revetment armor should be continuous for a distance which extends both upstream and downstream of the region which experiences hydraulic forces severe enough to cause dislodging and/or transport of bed or bank material. The minimum distances recommended are an upstream distance of 1.0 channel width and a downstream distance of 1.5 channel widths. The channel reach which experiences severe hydraulic forces is usually identified by site inspection, examination of aerial photography, hydraulic modeling, or a combination of these methods.
Many site-specific factors have an influence on the actual length of channel that should be protected. Factors that control local channel width (such as bridge abutments) may produce local areas of relatively high velocity and shear stress due to channel constriction, but may also create areas of ineffective flow further upstream and downstream in "shadow zone" areas of slack water. In straight reaches, field reconnaissance may reveal erosion scars on the channel banks that will assist in determining the protection length required.
In meandering reaches, since the natural progression of bank erosion is in the downstream direction, the present limit of erosion may not necessarily define the ultimate downstream limit. FHWA's Hydraulic Engineering Circular No. 20, "Stream Stability at Highway Structures" (Lagasse et al. 2001a) provides guidance for the assessment of lateral migration. The design engineer is encouraged to review this reference for proper implementation.
Vertical Extent. The vertical extent of the revetment should provide freeboard above the design water surface. A minimum freeboard of 1 to 2 ft should be used for unconstricted reaches and 2 to 3 ft for constricted reaches. If the flow is supercritical, the freeboard should be based on height above the energy grade line rather than the water surface. The revetment system should either cover the entire channel bottom or, in the case of unlined channel beds, extend below the bed far enough so that the revetment is not undermined by the maximum scour which for this application is considered to be toe scour, contraction scour, and long-term degradation (Figure 10.5).
Recommended revetment termination at the top and toe of the bank slope are provided in Figures 10.4 and 10.5 for armored-bed and soft-bottom channel applications, respectively. Similar termination trenches are recommended for the upstream and downstream limits of the gabion mattress revetment. This problem is presented in English units only because proprietary gabion mattresses in the U.S. are manufactured and specified in units of inches and pounds.
The following example illustrates the gabion mattress design procedure using the method presented in Section 10.3.1. The example is presented in a series of steps that can be followed by the designer in order to select the appropriate thickness of the gabion mattress based on a pre-selected target factor of safety. The primary criterion for product selection is if the computed factor of safety for the armor meets or exceeds the pre-selected target value. This problem is presented in English units only because gabion mattresses in the U.S. are manufactured and specified in units of inches and pounds.
A gabion mattress system is proposed to arrest lateral migration on the outside of a bend. The channel dimensions and design hydraulic conditions are given in Table 10.3.
Step 1. Determine a target factor of safety for this project:
Use Figure 10.3 to compute a target factor of safety. For this example, a target factor of safety of 1.7 is selected as follows:
The target factor of safety for this project site is calculated as:
SFT = (SFB)(XC)(XM) = 1.7
Step 2. Calculate design shear stress
The maximum bed shear stress at the cross section is calculated using Equation 10.1:
τdes = Kb(γ)(y)(Sf)
First calculate Kb using Equation 10.2:
Since Rc/T = 750/120 = 6.25
Kb = 2.38 - 0.206(6.25) + 0.0073(6.25)2 = 1.38
so τdes = 1.38 (62.4 lb/ft3) (5.0 ft) (0.005 ft/ft) = 2.15 lb/ft2
Step 3. Calculate permissible shear stress
From Equation 10.3,
Assuming a specific gravity of 2.65 for the stone fill, the unit weight of the individual stones is 2.65 x (62.4) = 165 lb/ft3. Using the recommended value of 0.10 for Cs, the permissible shear stress is plotted as a function of the d50 size of the stone fill in Figure 10.6:
Using a d50 stone size of 4.5 in., a permissible shear stress is calculated using Equation 10.3:
τp = 0.10 (165 lb/ft3 - 62.4 lb/ft3) (4.5 in / 12 in/ft) = 3.85 lb/ft2
Step 4. Calculate factor of safety:
Using Equation 10.4, the factor of safety is calculated as:
= = 1.8
Since the calculated factor of safety is larger than the site-specific target factor of safety of 1.7 for this project, the stone sizing is appropriate.
Step 5. Specify the gabion mattress:
The thickness of the gabion mattress should be at least 2 times the d50 size of the stone fill. For this project, select a mattress with a thickness of at least 2 x 4.5 in. = 9 in. A filter should be provided beneath the gabion mattress designed in accordance with the procedures described in Design Guide 16 of this document.
The hydraulic stability of gabion mattresses at bridge piers can be assessed using the factor of safety method as previously discussed. However, uncertainties in the hydraulic conditions around bridge piers warrant increasing the factor of safety in lieu of a more rigorous hydraulic analysis. Experience and judgment are required when quantifying the factor of safety to be used for scour protection at an obstruction in the flow. In addition, when both contraction scour and pier scour are expected, design considerations for a pier mat become more complex. The following guidelines reflect guidance from NCHRP Report 593, "Counter-measures to Protect Bridge Piers from Scour" (Lagasse et al. 2007).
The issues involved in selecting a target factor of safety for designing gabion mattresses for pier scour protection are described in Section 10.3.2, and illustrated in flow chart fashion in Figure 10.3. Note that for bridge scour applications, the minimum recommended factor of safety is 1.5, as compared to a value of 1.2 for typical bank revetment and bed armor applications.
The design hydraulic conditions in the immediate vicinity of a bridge pier are more severe than the approach conditions upstream. Therefore, at a pier, the local velocity and shear stress should be used in the design equations. As recommended in NCHRP Report 593, the section-average approach velocity V must be multiplied by factors that are a function of the shape of the pier and its location in the channel:
If the velocity distribution is available from stream tube or flow distribution output from a 1-D model, or directly computed from a 2-D model, then only the pier shape coefficient should be used to determine the design velocity. The maximum velocity in the active channel Vmax is recommended since the channel could shift and the maximum velocity could impact any pier:
The local shear stress at the pier,τdes, is calculated using a rearranged form of Manning's equation:
Based on small-scale laboratory studies performed for NCHRP Project 24-07(2)(Lagasse et al. 2007), the optimum performance of gabion mattresses as a pier scour countermeasure was obtained when the mattresses were extended a distance of at least two times the pier width in all directions around the pier.
In the case of wall piers or pile bents consisting of multiple columns where the axis of the structure is skewed to the flow direction, the lateral extent of the protection should be increased in proportion to the additional scour potential caused by the skew. While there is no definitive guidance for pier scour countermeasures, it is recommended that the extent of the armor layer should be multiplied by a factor Kα, which is a function of the width (a) and length (L) of the pier (or pile bents) and the skew angle α as given below (Richardson and Davis 2001):
Gabion mattresses should be placed so that the long axis is parallel to the direction of flow (Yoon 2005). Where only local scour is present, the gabion mattresses may be placed horizontally such that the top of the mattress is flush with the bed elevation; however, when other types of scour are present, the mattressesmust be sloped away from the pier in all directions such that the depth of the system at its periphery is greater than the maximum scour which for this application is considered to be the depth of contraction scour and long-term degradation, or the depth of bedform troughs, whichever is greater (Figure 10.7). The mattresses should not be laid on a slope steeper than 1V:2H (50%). In some cases, this criterion may result in gabions being placed further than two pier widths away from the pier.
Methods of predicting bedform geometry can be found in Karim (1999) and also in van Rijn (1984). An upper limit on crest-to-trough height Δ is provided by Bennet (1997) as Δ < 0.4y where y is the depth of flow. This guidance suggests that the maximum depth of the bedform trough below ambient bed level is approximately 20% of the depth of flow.
A filter is typically required for gabion mattresses at bridge piers. The filter should not be extended fully beneath the gabions; instead, it should be terminated 2/3 of the distance from the pier to the edge of the gabion mattress. When using a granular stone filter, the layer should have a minimum thickness of 4 times the d50 of the filter stone or 6 in., whichever is greater. The granular filter layer thickness should be increased by 50% when placing under water.
Manufacturer's assembly instructions should be followed. Mattresses should be placed on the filter layer and assembled so that the wire does not kink or bend. Mattresses should be oriented so that the long dimension is parallel to the flow and internal diaphragms are perpendicular to the flow. Prior to filling, adjacent mattresses should be connected along the vertical edges and the top selvedges by lacing, fasteners, or spirally binding. Custom fitting of mattresses around corners or curves should be done according to manufacturer's recommendations.
Care should be taken during installation so as to avoid damage to thegeotextile or subgrade during the installation process. Mattresses should not be pushed or pulled laterally once they are on the geotextile. Preferably, the mattress placement and filling should begin at the upstream section and proceed downstream. If a mattress system is to be installed starting downstream and proceeding in the upstream direction, a contractor option is to construct a temporary toe trench at the front edge of the mattress system to protect against flow which could otherwise undermine the system during flow events that may occur during construction. On sloped sections where practical, placement and filling shall begin at the toe of the slope and proceed upslope.
Gabion mattresses placed in water require close observation and increased quality control to ensure a continuous countermeasure system. A systematic process for placing and continuous monitoring to verify the quantity and layer thickness is important.
Excavation, grading, and placement of gabion mattresses and filter under water require additional measures. For installations of a relatively small scale, diversion of the stream around the work area can be accomplished during the low flow season. For installations on larger rivers or in deeper water, the area can be temporarily enclosed by a cofferdam, which allows for construction dewatering if necessary. Alternatively, a silt curtain made of plastic sheeting may be suspended by buoys around the work area to minimize environmental degradation during construction.
Depending on the depth and velocity of the water, sounding surveys using a sounding pole or sounding basket on a lead line, divers, sonar bottom profiles, and remotely operated vehicles (ROVs) can provide some information about the mat placement and toedown.
The importance of the filter component of gabion mattress installation should not be underestimated. Geotextile filters are most commonly used with gabion mattresses, although coarse granular filters may be used where native soils are coarse and the particle size of the filter is large enough to prevent winnowing through the rock fill of the gabion mattresses. When using a granular stone filter, the layer should have a minimum thickness of 4 times the d50 of the filter stone or 6 in., whichever is greater. The d50 size of the granular filter should be determined by using the procedure presented in Design Guideline 16 of this document. When placing a granular filter under water, its thickness should be increased by 50%.
The filter must retain the coarser particles of the subgrade while remaining permeable enough to allow infiltration and exfiltration to occur freely. It is not necessary to retain all the particle sizes in the subgrade; in fact, it is beneficial to allow the smaller particles to pass through the filter, leaving a coarser substrate behind. Detailed aspects of filter design are presented in Design Guideline 16 of this document.
Some situations call for a composite filter consisting of both a granular layer and a geotextile. The specific characteristics of the base soil determine the need for, and design considerations of the filter layer. In cases where dune-type bedforms may be present at the toe of a bank slope protected with gabion mattresses, it is strongly recommended that only a geotextile filter be considered.
Sand-filled geotextile containers made of nonwoven needle punched fabric are particularly effective for placement under water as shown in Figure 10.8. The fabric for the geotextile containers should be selected in accordance with the filter design criteria presented in Design Guideline 16, and placed such that they overlap to cover the required area. Geotextile containers can be fabricated in a variety of dimensions and weights. Each geotextile container should be filled with sand only to about 50 to 65% of the container's total volume so that it remains flexible and "floppy." The geotextile containers can also serve to fill a pre-existing scour hole around a pier prior to placing the gabion mattresses, as shown in Figure 10.7. For more detail, see Lagasse et al. (2007).
An observed key point of failure for gabion mattress systems at bridge piers during laboratory studies occurs at the joint where the mat meets the bridge pier. During NCHRP Project 24-07(2), securing the geotextile to the pier prevented the leaching of the bed material from around the pier. This procedure worked successfully in the laboratory, but there are constructability implications that must be considered when using this technique in the field, particularly when placing the mattress under water.
A grout seal between the mattress and the pier is recommended. A grout seal is not intended to provide a structural attachment between the mattress and the pier, but instead is a simple method for plugging gaps to prevent bed sediments from winnowing out between the mattress and the structure. In fact, structural attachment of the mattress to the pier is strongly discouraged. The transfer of moments from the mat to the pier may affect the structural stability of the pier, and the potential for increased loadings on the pier must be considered. When placing a grout seal under water, an anti-washout additive is required.
Anchors are not typically used with gabion mattress systems; however, the layout guidance presented in Section 10.4 indicates that the system should be toed down to a termination depth at least as deep as any expected contraction scour and long-term degradation, or bedform troughs, whichever is greater. Where such toe down depth cannot be achieved, for example where bedrock is encountered at shallow depth, a gabion mattress system with anchors along the front (upstream) and sides of the installation are recommended. The spacing of the anchors should be determined based on a factor of safety of at least 5.0 for pullout resistance based on calculated drag on the exposed leading edge. Spacing between anchors of no more than 4 ft is recommended. The following example is provided:
Step 1: Calculate total drag force Fd on leading edge of system:
Fd = 0.5ρV2(Δz)(b) = 0.5(1.94)(102)(0.5)(40) = 1,940 lbs
Step 2: Calculate required uplift restraint using 5.0 safety factor:
Frestraint = 5.0(1,940) = 9,700 lbs
Step 3: Counting anchors at the corners of the system, calculate required pullout resistance per anchor:
Anchors should never be used as a means to avoid toeing the system down to the full required extent where alluvial materials are present at depth. In this case, scour or bedform troughs will simply undermine the anchors as well as the system in general.
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